This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 9900289-1 filed in Sweden on Jan. 27, 1999; the entire content of which is hereby incorporated by reference.
The present invention relates generally to a homodyne radio receiver apparatus and more particularly to a homodyne radio receiver apparatus, comprising DC compensation means for determining a DC-level estimate of a received signal burst.
Homodyne receivers are well known for use in wireless telecommunication devices, such as mobile telephones.
Generally, a homodyne receiver comprises dual communication channels, which are commonly known as I and Q channels. The dual-channel homodyne receiver is used in prior art digital mobile telephones, such as GSM phones.
The receiver comprises an antenna for receiving an incoming electromagnetic communication signal, such as a TDMA signal (xe2x80x9cTime Division Multiple Accessxe2x80x9d) representing a stream of digital data symbols, which have been modulated onto two orthogonal carrier waves. The received signal is fed through a bandpass filter, amplified in an amplifier and then split into two identical signals. The first of these signals goes to a first signal path, where it is initially mixed in a mixer with an intermediate frequency signal. The intermediate frequency signal is fed from a local oscillator and passes unmodified through a phase shifter. Similarly, the second signal goes to a second signal path, where it is mixed in a mixer with the intermediate frequency signal from the local oscillator, once the phase of the intermediate frequency signal has been shifted by 90xc2x0 in the phase shifter.
The output of the mixer is filtered by a lowpass filter and amplified in a second amplifier. Subsequently, the signal is fed to an analog-to-digital converter for sampling the signal and converting it to a digital signal comprising the stream of data symbols. The digital signal is filtered in a digital lowpass filter, and the digital data symbols contained in the signal are supplied to a digital memory. An estimated DC-level of the signal is determined and subtracted from the digital signal forming a DC-level compensated digital signal, which is demodulated.
The stream of digital data symbols are subsequently used by other components in the mobile telephone for producing e.g. an audible output through a loudspeaker.
However, it has been found that prior art homodyne radio receivers have insufficient receiver performance, particularly for representing data messages sent between two computers during a data communication session.
The information bandwidth of a homodyne receiver goes down to DC in frequency, but the DC-level contains no information. Hence, the DC-level needs to be removed before the information can be demodulated properly.
The DC-level of a signal can be estimated in different ways. One way of estimating the DC-level is disclosed in U.S. Pat. No. 5,838,735, wherein the DC-level of a signal is determined by means of an average calculator, which calculates the mean value or the average DC-level of the digital signal according to the following expression:
DClevel=1/Nxcexa3ixe2x88x921NSxe2x80x83xe2x80x83(i)
Hence, the mean value of the samples S(i) is determined by forming the average of a predetermined number N of the most recently received samples. Then, the average DC-level is subtracted from the digital signal forming a DC-level compensated digital signal.
In TDMA systems like GSM it is difficult to determine the DC-level, because the time to measure the DC-level is limited to one received burst, since the next received burst can have another DC-level due to for instance frequency hopping. The number of samples included in an average calculation is for example 128 bits. This implies the assumption that, the average of 128 GMSK (Gaussian Minimum Shift Keying) modulated I samples (or Q samples) is zero. However, averages equal to zero is only the case when signal points are positioned equally frequent in all quadrants in the IQ plane.
An investigation shows that the modulation in some cases causes a DC offset of more than 20% (typically 10%) of the amplitude for 128 samples. Such, DC-level estimates causes a limited performance for high C/I (carrier-to-interference ratio) and SNR (signal-to-noise ratio) signals, and an unsatisfying number of bit errors, particularly when the signals are uses for transmitting data during a data communication session. Of course, the accuracy in the DC-level estimate is also dependent on the modulation type.
An object of the present invention is to provide a homodyne radio receiver apparatus and method, providing an improved DC-level estimate and compensation.
This object is achieved by a homodyne radio receiver apparatus according to the invention, wherein a radio frequency signal burst of a number of symbols is received in the apparatus and converted to digital samples for DC compensation by a DC compensator. A first estimate of the DC-level is determined by calculating an average DC-level of the samples in an average calculator.
Then, according to one aspect of the invention, a signed square sum of a plurality of the samples are calculated by a signed square sum calculator in the DC compensator. The square sum is weighted by a compensation factor, and correction means connected to the square sum calculator subtracts the weighted square sum from the average DC-level for forming an improved DC-level estimate.
According to another aspect of the invention, the DC compensation means comprises a signed sum calculator for calculating a signed sum of a plurality of the samples and weighting the sum by a compensation factor. Correction means is also included in the DC compensation means, wherein an output of the signed sum calculator means is subtracted from an output of the average calculator means for forming the DC-level estimate.
An introduction of a DC compensator, which estimates a DC-level as described and compensates the signal with a parameter dependent on the signal shape, the output from the function will become correlated with the error in the DC-level estimate. The function extracts information from the signal, which can be used to compensate for any error in the DC-level estimate.
An advantage of the present invention is that the DC-level estimate and compensation according to the invention removes the limiting DC-level error for high C/I (carrier-to-interference ratio) and SNR (signal-to-noise ratio) signal conditions, by considering information characteristics of the signal for DC-level estimation, resulting in a reduced number of bit errors for the current signal conditions.